How do I fill a texture with another texture?
Easiest way how to blend textures is mix them together using "Colors" -> "MixRGB"  node in node editor:


Second option is use external image editor Like GIMP, Krita, Photoshop etc.
For fast and easy workflow you can set path to your image editor in 

"File: -> "User Preferences" -> "File" -> "Image editor"

Now you can send your texture to your external image editor from Blender UV/Image editor by 

"Image" -> "Edit externaly". 

If you finish editing in external Image editor, just overwrite original image and In Blender Image Editor use 

"ALT+R" for reload your edited image.